Substrate coated with a composition of a blend of a glycidyl acrylic polymer and a reactive polysiloxane

ABSTRACT

A substrate coated with a dried coalesced composition of a liquid carrier and a binder of a blend of 
     a. an acrylic polymer containing glycidly groups, and 
     b. a crosslinkable polysiloxane having attached to the silicone atoms of its backbone alkyl groups, phenyl groups and hydroxyl groups; 
     optionally, a fluorocarbon polymer can be added to the composition; 
     the composition cures at ambient temperatures and provides a finish to metal substrates that is durable, weatherable and heat resistant and is particularly useful for coating pipes, tanks, vessels and stacks of plants and refineries that are subjected to weathering, chemicals and heat.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of Ser. No. 430,840 filed Sept. 30, 1982now U.S. Pat. No. 4,446,259.

BACKGROUND OF THE INVENTION

This invention relates to an ambient temperature curable coatingcomposition which forms a durable, weatherable chemical and temperatureresistant finish.

The prior art shows coating compositions of acrylic silicone copolymerswhich are crosslinked by heat or radiation after application to asubstrate. Patents showing such compositions are as follows: Sekmakas,U.S. Pat. No. 3,468,836, issued Sept. 23, 1969, Kincheloe et al., U.S.Pat. No. 3,644,566 issued Feb. 22, 1972, Nordstrom et al., U.S. Pat. No.3,650,813 issued Mar. 21, 1972 and Sekmakas U.S. Pat. No. 3,655,602issued Apr. 11, 1972.

In the construction or repair of industrial chemical plants and oilrefineries, coatings are applied to substrates such as pipes, stacks,tanks, reactors and the like. These coatings must be applied at ambienttemperatures, provide protection to the substrate from moisture andweathering before the plant is placed in operation. After the plant isin operation, these coatings are subjected to chemicals and elevatedtemperature conditions of 100°-200° C. and above and the coatings mustremain intact and provide protection to the substrates at these elevatedtemperatures. The aforementioned prior art compositions are inadequatefor the above purpose since these compositions must be heated toelevated temperatures or cured by radiation to form a durable coating.This would be impossible or very impractical for new plant constructionwhere the plant would not be operational for a long period of time or inan old plant which generally is painted when the plant is not inoperation.

There is a need for a coating composition that does not require curingat an elevated temperature but will cure at ambient temperatures to forma weatherable, durable and chemical resistant finish that will withstandhigh temperatures which occur during plant and refinery operations.

SUMMARY OF THE INVENTION

A substrate coated with a dried coalesced layer resulting from acomposition containing about 10-80% by weight of a film-forming binderand 20-90% by weight of a liquid carrier; wherein the binder is a blendof about

a. 20-90% by weight, based on the weight of the binder, of an acrylicpolymer of about 10-50% by weight, based on the weight of the acrylicpolymer, of polymerized glycidyl methacrylate or glycidyl acrylate and50-90% by weight of other polymerized ethylenically unsaturated monomerssuch as alkyl methacrylates, alkyl acrylates, styrene or alkylsubstituted styrenes; wherein the acrylic polymer has a weight averagemolecular weight of about 10,000-100,000 determined by gel permeationchromatography using polymethylmethacrylate as a standard and

b. 10-80% by weight, based on the weight of the binder, of acrosslinkable polysiloxane having attached to a silicone atoms of itsbackbone alkyl groups with 1-6 carbon atoms, phenyl groups and hydroxylgroups and containing sufficient hydroxyl groups to provide a silanolcontent of about 0.5-7% by weight, based on the weight of thepolysiloxane.

DETAILED DESCRIPTION OF THE INVENTION

The coating composition used to form the coated substrate of thisinvention contains about 10-80% by weight of a film-forming binder and20-90% by weight of a liquid carrier. Preferably, the compositioncontains 40-70% by weight of the film-forming binder and 30-60% byweight of the liquid carrier. Generally, the liquid carrier is anorganic solvent for the binder. The carrier can be aqueous to form adispersion or a blend of a nonsolvent and solvent for the binder to forma nonaqueous dispersion.

The binder of the composition is a blend of 20-90% by weight of anacrylic polymer and 10-80% by weight of a polysiloxane. Preferably, thecomposition contains about 50-75% by weight of the acrylic polymer and25-50% by weight of the polysiloxane.

In general, the acrylic polymer must be compatible with the polysiloxaneand contain a sufficient number of reactive groups, primarily glycidylgroups, to crosslink with the polysiloxane under ambient temperaturesafter the coating compositions has been applied to a substrate. Theacrylic polymer should have a glass transition temperature of about 20°C. to 50° C. A number average molecular weight of about 2,000 to 20,000,a weight average molecular weight of about 10,000 to 100,000 and amolecular weight distribution of about 2 to 5.

The molecular weight of the acrylic polymer is determined by gelpermeation chromatography using polymethylmethacrylate as a standard.

The glass transition temperature of the polymer is determined bydifferential scanning colorimetry or is calculated.

The acrylic polymer is prepared by conventional polymerization procedurein which monomers, catalyst and solvent are changed into a conventionalpolymerization vessel and reacted at about 60° to 175° C. for about 1-6hours to form the polymer.

Typical solvents which are used to prepare the acrylic polymers are thefollowing: toluene, ethyl acetate, acetone, methyl isobutyl ketone,methylethyl ketone, ethyl alcohol, mineral spirits, ethylene glycolmonoethyl ether acetate, and other aliphatic, cycloaliphatic andaromatic hydrocarbon, esters, ethers, ketones and alcohols which areconveniently used.

About 0.1-4% by weight, based on the weight of the monomers, of thepolymerization catalyst is used to prepare the acrylic polymer. Typicalcatalysts are azobis-isobutyronitrile, azo-bis(gamma dimethylvaleronitrile), benzoyl peroxide, t-butyl pivalate and the like.

The acrylic polymer contains about 10-50% by weight of glycidylmethacrylate or glycidyl acrylate and 50-90 % by weight of otherethylenically unsaturated monomers. Preferably, the acrylic polymercontains about 10-30% by weight of glycidyl methacrylate or glycidylacrylate and 70-90% by weight of other ethylenically unsaturatedmonomers.

Typical ethylenically unsaturated monomers that are used to form theacrylic polymer are as follows: alkyl methacrylates having 1-12 carbonatoms such as methyl methacrylate, ethyl methacrylate, butylmethacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, nonylmethacrylate, lauryl methacrylate, cyclohexyl methacrylate, isodecylmethacrylate, propyl methacylate, phenyl methacrylate, isobornylmethacrylate and the like; alkyl acrylates having 1-12 carbon atoms inthe alkyl group such as methyl acrylate, ethyl acrylate, propylacrylate, isopropyl acrylate, butyl acrylate, isobutyl acrylate, hexylacrylate, 2-ethylhexyl acrylate, nonyl acrylate, lauryl acrylate,cyclohexyl acrylate, isodecyl acrylate, phenyl acrylate, isobornylacrylate and the like; styrene, alkyl substituted styrene such asα-methyl styrene, t-butyl styrene; vinyl toluene.

One preferred acrylic polymer contains about 10-30% by weight ofglycidyl methacrylate, 10-20% by weight of styrene, 50-60% by weight ofbutyl methacrylate and 5-20% by weight butyl acrylate. One particularlypreferred acrylic polymer contains 20% by weight glycidyl methacrylate,15% by weight styrene, 55% by weight butyl methacrylate and 10% byweight butyl acrylate.

The polysiloxane contains the following units ##STR1## where R and R¹are alkyl group having 1-6 carbon atoms, phenyl group or hydroxy groupand contains sufficient number of hydroxyl groups to provide a silanolcontent of about 0.5-7% by weight. One preferred resin is Dow Corning'sD.C. 840 silicone resin which formed by hydrolyzing selected amounts ofmono, di and tri methyl chlorosilicone, mono, di and tri phenylchlorosilicone, mono, di and tri propyl chlorosilicone and mono, di andtri amyl chloro-silicone and the resulting products are condensed toform a resin blend having a silanol content of about 1.9-3% by weight.Other polysiloxanes which give similar results are Bayer's "Baysilone"Resin P150K, Rhone-Poulenc's "Rhodorsil" 6406 X and General Electric'sSR-165. These polysiloxanes are proprietary products but are believed tobe formed as described above.

Generally, the composition contains pigments in a pigment to binderweight ratio of about 10/100 to 300/100. Often coatings formed from thecomposition are exposed to high temperatures and inorganic pigments areused such as metal oxides like titanium dioxide, iron oxide, and thelike; metallic pigments such as aluminum flake, nickle flake andpowdered aluminum; sulfates, silica talc, and the like.

Trace amounts of multivalent metal ions of iron, aluminum, zinc and tinfrom the above pigments are present in the composition and co-ordinatewith the silanol groups of the polysiloxane and catalyze thecrosslinking reaction between the acrylic polymer and polysiloxane toprovide ambient temperature curing of the coating composition afterapplication to a substrate. If the metal ions complexed with the silanolgroups are not present, elevated temperatures are required to cure thecomposition.

To insure stability of the coating composition during storage, compoundssuch as acetyl acetone are added in amounts of about 0.75-3% by weight.The acetyl acetone complexes with metal ions in the composition andprevents catalyzation of the crosslinking reaction during storage. Uponapplication the acetyl acetone volatilizes and allows the metal ions tocomplex with the silanol groups as described above and catalyze curingof the finish. Other auxiliary stabilizers can also be added such asglycidyloxy alkoxy silanes such as gamma-glycidoxy propyl trimethoxysilane.

The coating composition can be applied using conventional techniquessuch as spraying, brushing, roller coating, flow coating and the likeand dried at ambient temperatures to give a finish about 1-5 mils thick.The composition can be applied to a wide variety of metal substratessuch as iron, grit blasted steel or aluminum. Preferably, a primer of apolysilicate and inorganic zinc pigments is used. The resulting coatingis durable, weatherable, has good adhesion to primed and unprimedsubstrates and is heat and chemical resistant. These characteristicsmake the composition particularly useful for painting pipes, tanks,stacks, and reactors of a chemical plant or refinery before the plant orrefinery is placed in operation. The coating provides good protectionfrom weathering before the plant or refinery is placed in operation andcontinues to provide good protection after the plant or refinery isplaced in operation even when these coatings are exposed to elevatedtemperatures of typical processes.

Another aspect of this invention is the addition of up to 30% by weight,based on the weight of the binder, of a fluorocarbon polymer to thecomposition to improve flexibility of the resulting finish, improvechemical resistance and reduces the coefficient of friction of thefinish. Generally, about 5-30% by weight of the fluorocarbon polymer isused. Typical fluorocarbon polymers that can be added are polyvinylidinefluoride, polytetrafluoroethylene, copolymers of tetrafluoroethylene andhexafluoroethylene, copolymers of ethylene and tetrafluoroethylene,copolymers of tetrafluoroethylene and perfluoropropyl vinyl ether andthe like.

These fluorocarbon polymer containing compositions can be applied by anyof the aforementioned techniques to any of the above substrates and thenthe resulting coating is baked at 200° to 345° C. for about 0.5 to 2hours to provide a coating 1 to 3 mils thick. These coatings have goodadhesion to substrates, good hardness and gloss, solvent resistance andhave good weatherability, excellent heat resistance and low coeficientof friction.

These compositions can be used to coat typical smaller parts of a plant,refinery or machinery that can be baked in an oven and are those partsthat are exposed to high temperatures and often in combination withweathering.

When polytetrafluoroethylene is used in the composition, the resultingcoating can be subjected to temperatures sufficiently high to oxidizethe constituents of the coating and the polytetrafluoroethylene willfuse to form a coating that is adhered to substrate and is still durableand weatherable. This composition is particularly useful as a coatingfor surfaces requiring resistance to severe chemical exposure andprovide low friction surface.

The following examples illustrate the invention. All parts andpercentages are on a weight basis. Molecular weights are determined bygel permeation chromatography using polymethyl methacrylate as astandard.

EXAMPLE I

An acrylic polymer solution was prepared by charging the followingconstituents into a conventional polymerization vessel equipped with astirrer, addition funnel, thermometer and a heating mantel:

    ______________________________________                                                           Parts by                                                                      Weight                                                     ______________________________________                                        Portion 1                                                                     Toluene              209.57                                                   Acetone              64.48                                                    Styrene monomer      66.49                                                    Butyl acrylate monomer                                                                             44.33                                                    Butyl methacrylate monomer                                                                         243.81                                                   Glycidyl methacrylate monomer                                                                      88.66                                                    Portion 2                                                                     Azo-bis-isobutyronitrile                                                                           4.43                                                     Toluene              22.16                                                    Acetone              22.16                                                    Portion 3                                                                     Azo-bis-isobutyronitrile                                                                           2.22                                                     Toluene              11.09                                                    Acetone              11.09                                                    Portion 4                                                                     Azo-bis-isobutyronitrile                                                                           2.22                                                     Toluene              11.09                                                    Acetone              11.09                                                    Total                814.89                                                   ______________________________________                                    

Portion 1 was charged into the polymerization vessel and heated to 85°C. and the heat was turned off. Portion 2 was added at a uniform rateover 30 minutes. During the addition, the temperature rose to about 95°C. After portion 2 was added, the resulting reaction mixture was held atits reflux temperature for 90 minutes and heating was continued tomaintain 85° C. temperature. Portion 3 was added at a uniform rate overa 15 minute period and then the reaction mixture was held at its refluxtemperature for 60 minutes. Portion 4 was added at a uniform rate over a15 minute period and the reaction mixture was held at is refluxtemperature for an additional 60 minutes.

The resulting acrylic polymer solution had a weight solids of 55%, thepolymer contained about 15% styrene, 10% butyl acrylate, 55% butylmethacrylate, and 20% glycidyl methacrylate, had a number averagemolecular weight weight of 15,300, a weight average molecular weight of42,400.

The following mill bases were prepared:

    ______________________________________                                                           Parts by                                                                      Weight                                                     ______________________________________                                        Black Mill Base                                                               Acrylic Resin solution (prepared                                                                   482.72                                                   above)                                                                        Black Pigment (a mixture of                                                                        132.76                                                   chrome oxide, copper oxide                                                    magnesium oxide and molyb-                                                    denum oxide)                                                                  Xylene               265.52                                                   Total                881.00                                                   White Mill Base                                                               Acrylic Resin solution (prepared                                                                   397.46                                                   above)                                                                        Titanium dioxide pigment                                                                           655.15                                                   Xylene               218.39                                                   Total                1271.00                                                  Brown Mill Base                                                               Acrylic Resin solution (prepared                                                                   479.43                                                   above)                                                                        Golden Brown Pigment (Mixture                                                                      131.86                                                   of iron oxide, aluminum oxide                                                 and titanium dioxide)                                                         Xylene               263.81                                                   Total                875.00                                                   ______________________________________                                    

Each of the above mill bases was charged into a conventional sand milland ground to form a uniform dispersion.

A black paint was prepared by mixing together the followingconstituents.

    ______________________________________                                                             Parts by                                                                      Weight                                                   ______________________________________                                        Acrylic Resin solution (prepared                                                                     334.90                                                 above)                                                                        Dow Corning's D.C. 840 proprietary                                                                   198.42                                                 polysiloxane solution (believed                                               to be 60% solids in toluene of                                                a blend of polysiloxanes which                                                are formed by the hydrolysis of                                               select amounts of mono, di and                                                tri methyl, propyl, amyl and                                                  phenyl chlorosilicones which                                                  are condensed to form a resin                                                 having a silanol content of                                                   about 1.9-3%.)                                                                Gamma-glycidoxypropyl  3.72                                                   trimethoxysilane                                                              Black mill base (prepared                                                                            271.60                                                 above)                                                                        Toluene                6.07                                                   Methanol               23.88                                                  Acetyl acetone         8.34                                                   Total                  846.93                                                 ______________________________________                                    

A tan paint was prepared by mixing together the following constituents:

    ______________________________________                                                          Parts by                                                                      Weight                                                      ______________________________________                                        Acrylic Resin Solution                                                                            205.93                                                    (prepared above)                                                              Polysiloxane Solution                                                                             155.77                                                    (described above)                                                             Gamma-glycidoxypropyl                                                                             3.12                                                      trimethoxy silane                                                             White Mill Base (prepared                                                                         426.20                                                    above)                                                                        Black Mill Base (prepared                                                                         7.17                                                      above)                                                                        Brown Mill Base (prepared                                                                         125.87                                                    above)                                                                        Methanol            27.73                                                     Toluene             21.50                                                     Acetyl acetone      9.71                                                      Total               983.00                                                    ______________________________________                                    

A can of each of the above paints was stored in an oven at 60°C. forfour weeks. The viscosity of the paint was measured after each week.There was no noticeable increase in the viscosity of the paints over thefour week period indicating that the paints have good shelf life.

Each of the paints was sprayed onto a primed steel panel and allowed todry at ambient temperatures. The resulting paint coatings were about1.5-2.5 mils thick, had good resistance to bases such as sodiumhydroxide and acids such as sulfuric acid, had acceptable gloss andappearance. Exposure of a panel to 80° C. increases film hardness to 14knoops and resistance to solvents such a methyl ethyl ketone. The panelswere exposed to a Q.U.V. accelerated weathering tester and after 1000hours showed no loss of gloss or film integrity and after 2500 hours ofexposure the coating on each panel was polished which brought the glossback to an acceptable level.

Separate sets of steel panels were prepared as above. One set wasexposed to 230° C. for an hour and a second set to 340° C. for one hour.The coatings on each of the panels discolored to an extent but did notcraze, crack or peel and remained intact. These panels were resistant toa 50% caustic solution and a 50% acid solution after several daysexposure.

EXAMPLE 2

The following coating compositions were prepared:

    ______________________________________                                                            Parts by                                                                      Weight                                                    ______________________________________                                        Composition 1                                                                 Acrylic Resin Solution (prepared                                                                    126.7                                                   in Example I)                                                                 Polysiloxanes Solution (described                                                                   35.9                                                    in Example I)                                                                 Polyvinylidene fluoride (powdered)                                                                  30.0                                                    Titanium dioxide pigment                                                                            50.0                                                    Methanol              7.3                                                     n-Propanol            7.3                                                     Toluene               70.1                                                    Total                 327.3                                                   Composition 2                                                                 Acrylic Resin Solution (prepared                                                                    126.7                                                   in Example I)                                                                 Polysiloxane Solution (described                                                                    35.9                                                    in Example I)                                                                 Polytetrafluoroethylene                                                                             30.0                                                    (micropowder)                                                                 Titanium dioxide pigment                                                                            50.0                                                    Methanol              7.3                                                     n-Propanol            7.3                                                     Toluene               70.1                                                    Total                 327.3                                                   Composition 3                                                                 Acrylic Resin Solution                                                                              126.7                                                   (described in Example I)                                                      Polysiloxane Solution 35.9                                                    (described in Example I)                                                      Polymer of tetrafluoroethylene/                                                                     30.0                                                    perfluoropropylvinyl ether                                                    Titanium dioxide pigment                                                                            50.0                                                    Methanol              7.3                                                     n-Propanol            7.3                                                     Toluene               70.1                                                    Total                 327.3                                                   Composition 4                                                                 Acrylic Resin Solution                                                                              126.7                                                   (described in Example 1)                                                      Polysiloxane Solution 35.9                                                    (described in Example 1)                                                      Ethylene/tetrafluoroethylene                                                                        30.0                                                    copolymer                                                                     Titanium dioxide pigment                                                                            50.0                                                    Methanol              7.3                                                     n-Propanol            7.3                                                     Toluene               70.1                                                    Total                 327.3                                                   Composition 5                                                                 Acrylic polymer Solution                                                                            126.7                                                   (described in Example 1)                                                      Polysiloxane Solution 35.9                                                    (described in Example 1)                                                      Copolymer of tetrafluoro-                                                                           30.0                                                    ethylene and hexafluoropropylene                                              Titanium dioxide pigment                                                                            50.0                                                    Methanol              7.3                                                     n-Propanol            7.3                                                     Toluene               70.1                                                    Total                 327.3                                                   ______________________________________                                    

Each of the compositions 1-5 were ground in a conventional ball mill toform a coating composition. Each composition was reduced to 55% weightsolids by the addition of a mixture of methanol and propanol.

A can of each of the compositions was stored in an oven at 60° C. forfour weeks as described in Example 1 and the viscosity of each wasmeasured at weekly intervals. There was no noticeable increase inviscosity of any of the compositions over the four week periodindicating that the compositions have a good shelf life.

Each of the compositions was sprayed into a primed steel panel and bakedat 235° C. for 60 minutes. In each case, the resulting coating was about1.5-2.5 mils thick, had good resistance to solvents such as methyl ethylketone, acceptable hardness and acceptable gloss and appearance.

The panels were exposed to Q.U.V. accelerated weathering tester andafter 2500 hours exposure the coatings did not craze, crack or peel butdid lose gloss.

Separate sets of steel panels were prepared as above. One set wasexposed to 230° C. for an hour, a second set to 340° C. for an hour anda third set exposed to 455° C. for an hour. The coatings on the panelsexposed to 230° C. and 340° C. discolored to an extent but did notcraze, crack or peel and remained intact. The panels coated withcompositions 1, 3, 4 and 5 exposed to 455° C. could be readily scrapedoff the panels but composition 2 containing polytetrafluoroethyleneadhered to the panel.

I claim:
 1. A substrate coated with a dried coalesced layer resultingfrom a coating composition comprising about 10-80% by weight of afilm-forming binder and 20-90% by weight of a liquid carrier selectedfrom the group consisting of organic solvent for the binder, aqueousliquid or a blend of solvent and nonsolvent for the binder; wherein thebinder consists essentially of a blend of abouta. 20-90% by weight,based on the weight of the binder, of an acrylic polymer consistingessentially of about 10-50% by weight, based on the weight of theacrylic polymer, of polymerized glycidyl methacrylate or glycidylacrylate and 50-90% by weight of other polymerized ethylenicallyunsaturated monomers selected from the group consisting of alkylmethacrylate having 1-12 carbon atoms, alkyl acrylate having 1-12 carbonatoms, styrene, alkyl substituted styrenes and mixtures thereof,andwherein the acrylic polymer has a weight average molecular weight ofabout 10,000-100,000, a number average molecular weight of about2,000-20,000 and a molecular weight distribution of about 2-5 and aglass transition temperature of about 20-50° C.; b. 10-80% by weight,based on the weight of the binder, of a crosslinkable polysiloxanehaving the following units: ##STR2## where R and R¹ are individuallyselected from the group consisting of alkyl group having 1-6 carbonatoms, hydroxyl group and phenyl group and contains sufficient hydroxylgroups to provide a silanol content of about 0.5-7% by weight.
 2. Thesubstrate of claim 1 in which the composition contains inorganicpigments in a pigment-to-binder weight ratio of about 10/100 to 300/100.3. The substrate of claim 2 in which the substrate is metal.
 4. Thesubstrate of claim 3 in which the composition contains 5 to 30% byweight, based on the weight of the binder, of a fluorocarbon polymer. 5.The substrate of claim 4 in which the fluorocarbon polymer ispolytetrafluoroethylene.
 6. The substrate of claim 4 in which thefluorocarbon polymer is a copolymer of tetrafluoroethylene andhexafluoroethylene.
 7. The substrate of claim 4 in which thefluorocarbon polymer is polyvinylidene fluoride.
 8. The substrate ofclaim 4 in which the fluorocarbon polymer is a copolymer of ethylene andtetrafluoroethylene.
 9. The substrate of claim 4 in which thefluorocarbon polymer is a copolymer of tetrafluoroethylene andperfluoropropyl vinyl ether.
 10. The substrate of claim 3 in which theacrylic polymer consists essentially of about10-30% by weight ofglycidyl methacrylate, 10-20% by weight of styrene, 50-60% by weight ofbutyl methacrylate and 5-20% by weight of butyl acrylate.
 11. Thesubstrate of claim 10 in which the acrylic polymer consists of20% byweight of glycidyl methacrylate, 15% by weight styrene, 55% by weight ofbutyl methacrylate, and 10% by weight of butyl acrylate.
 12. Thesubstrate of claim 3 in which the coating composition comprises about40-70% by weight of a film-forming binder and 30-60% by weight of anorganic solvent for the binder; in which the binder consists essentiallyof abouta. 50-75% by weight, based on the weight of the binder, of anacrylic polymer consisting essentially of about(1) 10-30% by weight,based on the weight of the acrylic polymer, of glycidyl methacrylate,(2) 10-20% by weight, based on the weight of the acrylic polymer, ofstyrene, (3) 50-60% by weight, based on the weight of the acrylicpolymer, of butyl methacrylate, and (4) 5-20% by weight, based on theweight of the acrylic polymer, of butyl acrylate; and b. 25-50% byweight, based on the weight of the binder, of a polysiloxane having thefollowing units: ##STR3## where R and R¹ are individually selected fromthe group consisting of methyl, propyl, amyl, hydroxyl and phenyl andthe polysiloxane contains a sufficient number of hydroxyl groups toprovide a silanol content of about 1.9-3.0%.
 13. The substrate of claim2 in which the composition contains about 0.75-3% by weight, based onthe weight of the composition, of acetyl acetone.
 14. The substrate ofclaim 13 in which the composition contains an aluminum pigment.
 15. Thesubstrate of claim 4 in which the composition has been fused at atemperature of at least 400° C.